This invention relates to a mass flowmeter operating by the Coriolis principle and incorporating a straight Coriolis measuring tube through which flows a fluid or medium, at least one oscillator associated with and exciting the Coriolis measuring tube, and at least one detector associated with the Coriolis measuring tube for capturing the Coriolis force values and/or the Coriolis-force-induced oscillations.
The above description states that the mass flowmeter discussed incorporates, inter alia, at least one oscillator xe2x80x9cassociatedxe2x80x9d with the Coriolis measuring tube, and at least one detector xe2x80x9cassociatedxe2x80x9d with the Coriolis measuring tube. It is common for the oscillator(s) or, in any event, part of the oscillator(s) and the detector(s) or, in any event, part of the detector(s) to be directly connected to the Coriolis measuring tube. However, since that is not absolutely necessary, the term xe2x80x9cassociatedxe2x80x9d is being used instead of xe2x80x9cconnectedxe2x80x9d.
For mass flowmeters operating by the Coriolis principle, one fundamentally distinguishes between those with an at least essentially straight Coriolis measuring tube and those with a looped Coriolis measuring tube. In the case of the mass flowmeters discussed here, one also distinguishes between designs employing only one Coriolis measuring tube and those with two Coriolis measuring tubes. Where two Coriolis measuring tubes are used, these may be connected in-line or positioned parallel to each other for the desired flow path.
In recent times, mass flowmeters with only one, essentially straight, Coriolis measuring tube have gained in popularity. Mass flowmeters operating by the Coriolis principle and equipped with one straight Coriolis measuring tube offer considerable advantages over mass flowmeters employing either two straight Coriolis measuring tubes or one looped Coriolis measuring tube. Compared to mass flowmeters with two straight Coriolis measuring tubes, their main advantage is that they obviate the need for a flow divider and a flow combiner, required in the case of mass flowmeters with two Coriolis measuring tubes. Compared to mass flowmeters employing one looped Coriolis measuring tube or two looped Coriolis measuring tubes, their main advantage lies in the fact that a straight Coriolis measuring tube is easier to manufacture than a looped Coriolis measuring tube, that in the case of a straight Coriolis measuring tube there is less of a pressure drop than in a looped Coriolis measuring tube, and that a straight Coriolis measuring tube can be cleaned more thoroughly than a looped Coriolis measuring tube.
Their advantages notwithstanding, mass flowmeters with only one straight Coriolis measuring tube also have drawbacks. For example, longitudinal expansion due to thermal effects can cause stress patterns in straight Coriolis tubes which, in extreme cases, may lead to mechanical damage to the Coriolis measuring tube, such as stress fissures and breaks. The reason is that in straight Coriolis measuring tubes, unlike for instance looped Coriolis measuring tubes, stress patterns caused by thermal expansion are not absorbed by a varied radius of curvature of the tube.
Another problem, albeit peculiar to all mass flowmeters operating by the Coriolis principle regardless of whether these mass flowmeters employ one Coriolis measuring tube or several Coriolis measuring tubes and regardless of whether the Coriolios measuring tubes are straight or looped, consists in the fact that, depending on the material used for the Coriolis measuring tube(s), chemical substances which would tend to corrode that material cannot be measured in the Coriolis mass flowmeter concerned. This might possibly impose severe limitations on the range of applications of the individual Coriolis mass flowmeter, necessitating the use of a different type of Coriolis mass flowmeter, meaning the replacement of the built-in Coriolis mass flowmeter.
In view of the above, it is the objective of this invention to provide a mass flowmeter, operating by the Coriolis principle, with one Coriolis measuring tube which displays only minor thermal expansion and corresponding stress patterns while at the same time offering high chemical resistance to corrosive substances.
The mass flowmeter according to this invention which solves the above-mentioned problem is characterized in that the Coriolis measuring tube consist of a ceramic material. A Coriolis measuring tube made from a ceramic material offers the advantage of permitting operation in a very wide temperature range including very high temperatures, displaying only moderate thermal expansion throughout the said wide operational temperature range. At the same time, ceramic materials are not affected, or temperatures, displaying only moderate thermal expansion throughout the said wide operational temperature range. At the same time, ceramic materials are not affected, or barely so, by corrosive substances such as chloric gases or liquids, which opens up a broad spectrum of possible applications for the Coriolis flowmeter according to this invention.
It is basically possible to use virtually any ceramic material for the Coriolis measuring tube in the Coriolis-type mass flowmeter according to this invention. Particular preference, however, is given to ceramic materials with especially high chemical resistance and with a low thermal expansion coefficient. Preferably, then, the Coriolis measuring tube consists of zirconium oxide or aluminum oxide and, according to a particularly preferred embodiment of this invention which allows the use of the Coriolis mass flowmeter for virtually all chemical compounds save for hydrofluoric acid (HF), of zirconium-stabilized aluminum oxide containing in excess of 5% zirconium. As an alternative, the Coriolis measuring tube for the mass flowmeter according to this invention preferably uses nitride ceramics.
The mass flowmeter of this invention, operating by the Coriolis principle, can be structured along essentially any conventional mass flowmeter design employing a single straight Coriolis measuring tube. However, in a preferred embodiment of this invention, a design is used whereby the mass flowmeter is provided with an outer enclosure which features a flange permitting installation in a pipe system. It is particularly desirable in this case to decouple the Coriolis measuring tube from any longitudinal forces in the pipe system in which it is installed. Such decoupling is preferably obtainable by firmly attaching the two ends of the Coriolis measuring tube to the outer enclosure while dimensioning and positioning it in such fashion that the Coriolis measuring tube is slightly set back from the lateral surfaces of the Coriolis mass flowmeter so that, when installed in the pipe system, it does not make direct contact with the latter.
As an alternative, it is also possible to connect only one end of the Coriolis measuring tube to the outer enclosure, allowing the Coriolis measuring tube to be longitudinally moved in relation to the outer enclosure. This approach serves as well to decouple the Coriolis measuring tube from the pipe system with respect to longitudinal forces.
To keep the Coriolis measuring tube in its proper position despite its longitudinal movability, elastic mounts are provided between the faces of the Coriolis measuring tube and the flange of the pipe system in which the Coriolis mass flowmeter can be installed, preferably in the form of O-ring gaskets which serve as an elastic support for the end section of the Coriolis measuring tube that is movable relative to the outer enclosure. In terms of the problem of longitudinal forces which are present in the longitudinal direction of, and bear on, the Coriolis measuring tube, it should be stated that ceramic components offer a certain resistance to longitudinal pressure while longitudinal tractive forces can much more readily lead to problems, meaning damage to the ceramic component. In view of this fact, the measures described above are intended to essentially decouple the Coriolis measuring tube from the longitudinal tractive forces.
In a further, preferred design embodiment of the Coriolis mass flowmeter according to this invention, at least one end of the Coriolis measuring tube is equipped with a firmly attached ring element which preferably consists of metal or a plastic material and by way of which the Coriolis measuring tube is connected to the outer enclosure. A ring of this type can serve several purposes: For one, the ring facilitates the attachment of the outer enclosure to the ceramic Coriolis measuring tube. For another, the ring can serve as a locating guide and support for a gasket. In addition, the ring element that is firmly attached to one end of the Coriolis measuring tube can help neutralize stress patterns in the ceramic Coriolis measuring tube which are caused by thermal expansion of the ceramic Coriolis measuring tube. If the Coriolis measuring tube were directly and solidly connected to the outer enclosure of the Coriolis mass flowmeter, the strength and substantial rigidity of the outer enclosure would not permit any changes in length of the ceramic Coriolis measuring tube without at least some stress arising in the Coriolis measuring tube. But a ring element firmly connected to the end of the Coriolis measuring tube, by virtue of a certain degree of elasticity, allows for at least a small, thermally induced longitudinal movement of the Coriolis measuring tube, eliminating or at least minimizing thermal-expansion-related stress patterns in the latter. Of course, the ring element that is firmly attached to one end of the Coriolis measuring tube will permit only longitudinal shifts of the Coriolis measuring tube which do not affect the symmetry and corresponding measuring accuracy of the Coriolis mass flowmeter.
The ring element can be attached to the end of the ceramic Coriolis measuring tube in a variety of ways. Preferably, however, the ring is crimped or shrink-mounted onto the ceramic Coriolis measuring tube. If the ring is of metal, it may also be welded onto a metallized surface section of the ceramic Coriolis measuring tube.
A further, preferred invention embodiment provides for the ceramic Coriolis measuring tube according to this invention to be impervious to gas and liquids by means of a seal between the end faces of the Coriolis measuring tube and the corresponding end faces of the flanges of the pipe system in which the Coriolis mass flowmeter is installed. Most preferably, this seal separates the Coriolis measuring tube from both the outer enclosure and the pipe system in which the Coriolis mass flowmeter is to be used. Also, a seal of this type when provided in the Coriolis mass flowmeter serves the additional purpose of absorbing and compensating for any thermally induced longitudinal expansion of the ceramic Coriolis measuring tube or of the pipe system and any possibly resulting compressive force exerted on the ceramic Coriolis measuring tube, thus essentially preventing such thermal expansion from engendering stress patterns in the Coriolis measuring tube. Most preferably, therefore, the seal consists of rubber or a plastic material, preferably Viton(copyright) or Kalrez(copyright).
According to a preferred further embodiment of this invention, the outer enclosure can be attached to the ends of the ceramic Coriolis measuring tube in simple, dependable and damage-free fashion by making the wall of the Coriolis measuring tube at least at one end thicker than in its mid-section. To ensure optimum oscillatory properties of the ceramic Coriolis measuring tube, the wall thickness in the mid-section of the ceramic Coriolis measuring tube is preferably less than 1 mm and is preferably about 0.7 mm.
In a preferred further development, an internal cylinder is attached to the ends of the Coriolis measuring tube in the mass flowmeter according to this invention. Here as well, it is desirable to attach the internal cylinder in an area of the ceramic Coriolis measuring tube in which the wall is thicker than in the mid-section whose wall thickness is reduced for optimal oscillation.
Finally, a preferred embodiment of the mass flowmeter according to this invention provides for the Coriolis measuring tube to be installable in the pipe system in such fashion that the Coriolis measuring tube is decoupled from the pipe system in terms of any bending force emanating from the pipe system and potentially affecting the Coriolis measuring tube. This can be accomplished, for instance, by providing at the ends of the Coriolis measuring tube an elastically deformable buffer ring between the Coriolis measuring tube and the outer enclosure which is elastically deformable in a direction which is essentially perpendicular to the longitudinal axis of the Coriolis measuring tube. Thus, any axial bending force of the pipe system that would be transferred to the mass flowmeter at an angle essentially perpendicular to the axis of the Coriolis measuring tube, while deforming the buffer ring, will for all practical purposes be absorbed without deforming the Coriolis measuring tube.